1. Field of the Invention
Exemplary embodiments of the present patent application relate to an image forming apparatus that incorporates a sheet feeding unit in which an uppermost sheet placed on a sheet stack is attracted to the surface of a dielectric belt by the action of an electric field generated by electric potential patterns formed on the surface of the dielectric belt and fed in a sheet feeding direction as the dielectric belt rotates.
2. Discussion of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a sheet of recording media according to image data. Thus, for example, a sheet feeding unit feeds a plurality of sheets one by one toward an image forming device. The image forming device forms an image on a sheet fed from the sheet feeding device.
The sheet feeding device incorporated in such related-art electrophotographic or inkjet image forming apparatuses often use a friction feed method by including a friction member to separate an uppermost sheet from other sheets of the sheet stack loaded in a sheet cassette. Specifically, the friction member, made of rubber having a high friction coefficient, pressingly contacts the uppermost sheet to separate the uppermost sheet from other sheets and conveys it as appropriate. One problem with such an arrangement is that the high friction coefficient of the friction member, which is necessary to feed the sheets to the image forming device in a stable manner, may deteriorate over time or according to environmental conditions, degrading feeding performance of the sheet feeding unit.
Further, when the image forming apparatus is used as a printer, it handles various types of recording media, such as plain paper, coated paper, and label paper. With recording media having a substantially small friction coefficient, sheets providing friction that varies depending on temperature, or sheets absorbing moisture and adhering to each other, the friction member of the sheet supplier may not separate the uppermost sheet from other sheets properly.
Further still, with recording media such as adhesive labels, the surface portion of the sheet can be easily separated from the underlying base layer of the sheet by the frictional force exerted between the pickup member and the recording medium, hindering reliable pick-up of the recording medium by the friction feeding method.
To address the above-described drawback, the image forming apparatus can employ an electrostatic sheet feed method in which recording media are electrically attracted to the surface of a dielectric belt by the action of an electric field generated by electric potential patterns formed on the surface of the dielectric belt and separated from a stack of recording media one by one as the dielectric belt rotates.
In the electrostatic sheet feed method, the electric potential patterns formed on the surface of the dielectric belt generate a non-uniform electric field at an interface between the surface of the dielectric belt and the upper surface of the sheet stack. The non-uniform electric field exerts a force of attraction in a normal direction of the interface based on Maxwell stress to convey the uppermost sheet placed atop the sheet stack as the dielectric belt rotates while attracting the uppermost sheet to the surface of the dielectric belt.
As an example of the electrophotographic image forming apparatus that employs such an electrostatic sheet feed method, Japanese Patent Application Publication No. 2003-237958 (JP-2003-237958-A1) has been proposed.
With the electrophotographic sheet feed method, if the uppermost sheet is picked up from the sheet stack on contacting the dielectric belt, several subsequent upper sheets including a second uppermost sheet are also sometimes picked up together with the uppermost sheet by the dielectric belt by action of an electric field generated by potential patterns formed on the dielectric belt. Therefore, the dielectric belt remains contacted with the sheet stack for a predetermined period of time from the moment the dielectric belt contacts the sheet stack before separating from the sheet stack, thus decreasing the action of the electric field on the second uppermost sheet, which in turn enables the uppermost sheet to be separated from the sheet stack. However, it is known that, for various reasons, the force of attraction is generated at the contact portion between the uppermost sheet and the second uppermost sheet even after the predetermined period of time elapses, and is consequently exerted over the uppermost sheet and the second uppermost sheet substantially to pick them up together.
To tackle the above-described drawback, JP-2003-237958-A discloses a sheet feeding device having a configuration in which the surface of the dielectric belt is effectively separated from the surface of the sheet stack to cause the dielectric belt to slope upward with respect to the surface of the sheet stack after attracting the uppermost sheet to the surface of the dielectric belt contacting the sheet stack.
In this configuration, as the dielectric belt moves away from the sheet stack, the uppermost sheet that is attracted to the surface of the dielectric belt is picked up from the sheet stack. At this time, although the second uppermost sheet is likely to follow the uppermost sheet, the rigidity of the second uppermost sheet provides a force of detachment for separating the second uppermost sheet from the uppermost sheet. Generally, the force of detachment is greater than the force of attraction at the contact portion between the uppermost sheet and the second uppermost sheet due to various reasons. Consequently, even if a force of attraction is generated, the uppermost sheet can be picked up successfully without being followed by the second uppermost sheet.
With the action of detachment, a space is formed in the contact portion between the uppermost sheet and the second uppermost sheet. Once this space is formed, it is easy to separate the uppermost sheet and the second uppermost sheet. Therefore, even if the force of attraction is generated at the contact portion between the uppermost sheet and the second uppermost sheet, the uppermost sheet can separate from the second uppermost sheet successfully.
(In this specification, the terms “pick-up operation” and “picking up” refers to the action or operation in which the dielectric belt attracts the uppermost sheet of the sheet stack thereto to bring the uppermost sheet upward and crate a gap between the uppermost sheet and the immediately underlying, adjacent sheet (i.e., the second uppermost sheet).)
However, in related-art sheet feeding devices for handling sheets including the above-described sheet feeding device disclosed in JP-2003-237958-A, a tensioned flat portion of the dielectric belt cannot form a sufficient angle with respect to the surface of the sheet stack (hereinafter “sheet pick-up angle”) when feeding the uppermost sheet that is attracted to the dielectric belt as the dielectric belt rotates. This is important because the greater sheet pick-up angle, the greater the restoring force that tends to restore the second uppermost sheet to its original flat shape. Consequently, the force of detachment generated by the sheet pick-up operation in which the second uppermost sheet is separated from the uppermost sheet also increases. Therefore, the related-art sheet feeding devices having a smaller sheet pick-up angle cannot provide a sufficient force of detachment to separate the second uppermost sheet from the uppermost sheet reliably.